1. Field of the Invention
The subject matter of the present invention pertains to an image distortion correction apparatus and to techniques for compensating for various types of image distortion appearing on a display.
2. Description of the Prior Art
When an electron beam scans across an inner face plate of a Cathode Ray Tube (CRT), while transferring video image information thereto, for display thereon, various types of video image distortion of the video image displayed on the CRT are produced. For example, as shown in FIG. 1, if the electron beam transfers the video image information to the CRT intermittently, at equally spaced intervals of time, during the scan thereof across said inner faceplate of said CRT, the video image information will be displayed thereon at non-equally spaced intervals of length or distance. Non-equally spaced intervals of length 10 are illustrated in FIG. 1. This type of distortion is called S-distortion. In three dimensions, the S-distortion of FIG. 1 takes a different form. This different form of distortion is called pin cushion distortion, illustrated in FIG. 2. U.S. Pat. No. 4,039,899, to Battjes et al., filed May 3, 1976 which describes the pin cushion distortion phenomenon. Another form of distortion is called horizontal linearity distortion, illustrated in FIGS. 3a and 3b. A DC voltage applied to a deflection yoke of the CRT tends to create a ramp current flowing therethrough. However, an inherent winding resistance, in series with the yoke, distorts the ramp current flowing therethrough, such that a distorted version thereof, as illustrated in FIG. 3a(3), is the result. The distorted ramp current, illustrated in FIG. 3a (3), distorts the video image information displayed on the CRT. The distorted video image, of this type, displayed on the CRT, is illustrated in FIG. 3b of the drawings.
A still further type of video image distortion is called anode loading, illustrated with reference to FIG. 4 of the drawings. When the anode supply voltage of the CRT is successively loaded (i.e. large beam currents), the anode voltage drops. The deflection sensitivity of said electron beam increases as a result thereof. Consequently, electron beam overscan is the result. This overscan is illustrated in the solid filled panel are of FIG. 4.
U.S. Pat. No. 4,039,899, referred to hereinabove, attempts to compensate for these distortions by predistorting a deflection signal by an amount necessary to provide an undistorted display. For example, in FIG. 5a, a horizontal and vertical ramp signal, generated by horizontal and vertical ramp signal generators, energize the X and Y inputs of a geometry circuit. A typical geometry circuit is illustrated in U.S. Pat. No. 4,039,899, referred to above, the disclosure of which is incorporated herein by reference. The geometry circuit pre-distorts the horizontal and vertical ramp signals by an amount necessary to compensate for the above-mentioned distortions. A pre-distorted corrected X-output signal and a pre-distorted corrected Y-output signal is generated therefrom for energizing via power amplifiers P an X-deflection coil and a Y-deflection coil, respectively, attached to the CRT. Since the horizontal and vertical ramp signals are pre-distorted prior to energization of the respective X and Y deflection coils, the above-mentioned distortions have been compensated therefor. The prior art also includes a switched X-axis drive of approximated predistortion via magnets, width coil, saturable reactor, and S-capacitor (C.sub.s), as seen in FIG. 5b.
However, a less costly, alternative method and apparatus is required to compensate for all of the above-mentioned types of video image information distortion.